Wmbdukes, the problem is, the Imperial Japanese Army did do human experimentation in real life, on prisoners of war and regular people (including women, some of them pregnant, and children) from occupied territories rounded up by the kempeitai (that was their Gestapo equivalent, sort of). It included things like human vivisection without anaesthesia, amputation of limbs to be reattached to the other side of the body, and injecting animal substances into people just to see what would happen.
Which is rather too close to "putting tentacles in place of arms FOR SCIENCE" for some of us, who find this shirt unsettling.
So yeah, it's not just satire. It's pretty close to real life war crimes, which were the stuff of nightmares.
Mind you, I like Japan. My nickname is from Japanese, my icon is a manga character, I've already been twice there and I was planning to do a countryside tour next March/ April so I can do some cherry blossom viewing if I'm lucky, but those things the Japanese Army did during WW2 were Sick and Wrong.